SAN FRANCISCO (Reuters) - The head of Microsoft Corp's $9 billion research unit thinks the debate over stopping climate change is being muddied by talk of renewable energy.

Efforts should be focused on stopping output of carbon dioxide, the greenhouse gas which helps heat the planet, rather than moving to a source that can be regenerated, said Craig Mundie, the man who replaced Bill Gates as the world's largest software maker's futurist.

Conservation and radical technologies, including new nuclear, could be key, he said.

"If you talk to the man on the street, there is a lot more awareness of the concept of renewable energy than there is about the need for a zero-carbon source," said Mundie, Microsoft Chief Research and Strategy Officer, in an interview on the sidelines of a major electricity conference.

Even industry insiders were not looking hard enough at "the scarier or less understood technological approaches," he said.

The U.S. Congress is considering a massive climate bill and nations globally are preparing to negotiate a follow up to the Kyoto climate change treaty at the end of this year. Development of solar power, wind and other 'clean' energy sources are hot topics globally.

But the fastest way to cut carbon output is to cut use, Mundie offered.

"The first crucial step we have to take across the board is conservation," he said in the speech at the Edison Electric Institute after introducing software for consumers to monitor their own power use, called Hohm.

He argued that humans will need energy alternatives that can be built in large arrays quickly to meet the demands of a growing, developing planet, and saw one such possibility as nuclear.

"I'm probably more enchanted with the idea of these novel nuclear approaches than any other one technology. There's wind, there's solar, there's tides, there's geothermal, but I think each of these is going to be more challenging to harness at some very large scale," he said, stressing that he was not giving his company's official policy.

He referred in particular to research work on traveling-wave reactors, a technology that aims to use slow-burning reactions in waste fuel from current plants.

"I think it would be a fascinating outcome if we woke up 10 years from now and said look, you can have all the nuclear you want, it doesn't have a weapons problem related to it, the cost of fuel is relatively low, there's no reprocessing requirements. It would be kind of a silver bullet for the energy problem."